The present invention relates to geological parameters determination and especially to Gaussian and pluri-Gaussian simulations of lithologies.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.
The estimation of an oil reservoir or a mining deposit usually comprises determination of geological parameters such as the facies composition of a subsoil.
Truncated Gaussian simulations have been first introduced in 1987 and have been commonly used in geological simulation to determine realistic facies repartition. These simulations are based on a random field truncation, therefore a truncation order that should be defined for each simulation.
With truncated Gaussian simulations, lithofacies are sorted for truncation (i.e. stratigraphic sequence): lithofacies that are not successors in the stratigraphic sequence will never be in contact. This feature has been proved to be too restrictive: for example, the facies may be impossible to order as there is no such order in the subsoil; moreover specific facies may be in contact with more than two facies.
To overcome these limitations, pluri-Gaussian simulations were proposed in 1994 and 1996. The basic idea behind pluri-Gaussian simulations is to start out by simulating more than one Gaussian random fields at every grid point in the study domain and to use some simple rule to convert these Gaussian values into lithotype indicators (or more generally into geological categorical property or facies).
In pluri-Gaussian models, a set of p Gaussian random fields are used to define the lithofacies structure. Most of the time p is set to 2 but similar works may be made with different values of p. The multi-variate Gaussian field is transformed into a multi-variate uniform field. Lithofacies are then assigned according to a truncation diagram (named also rock type rule or facies assignation diagram). Typical truncation diagrams separate the bi-variate uniform domain along vertical and horizontal lines. The precise layout or these lines defines the type of the chosen model.
These simulations are intended to model complex geology with different structure orientations and heterogeneous deposits (channels, reefs, bars, differently oriented facies, sets of conjugate veins or ore types where geological constraints apply, etc.). They may provide realistic and detailed images of internal structure.
These simulations may allow controlling the facies relationships and boundaries when dealing with complex geometrical configurations.
Nevertheless, such Gaussian or pluri-Gaussian simulations have drawbacks.
For example, it may be difficult to determine the relationship between Gaussian variables and physical processes. Moreover, the determination of the truncation diagram may not be easy and intuitive.
If non-stationary parameters are present in the model, the update of the truncation diagram, according to the non-stationary parameters, may be very complex. Therefore, person skilled in the art would prefer creating simplistic truncation diagrams (for instance, with rectangle domains) in order to be able to adapt them to target proportions.
There is thus a need for a method to determine complex (but adequate) truncation diagram in order to efficiently model and estimate facies repartition in a subsoil.